Drugged driving is an understudied issue, despite evidence that the prevalence of cannabis-involved drivers on America's roadway is raising. The potential that some states soon will legalize or further decriminalize cannabis use may exacerbate this. Yet the extent to which cannabis use impairs driving and elevates crash risk is not entirely clear. Although most crash-risk studies suggest significantly higher odds of crash involvement for cannabis-positive drivers, these findings are not universal. Further, even studies that do report elevated risk fail to accommodate the alternative explanation that the observed relationship between cannabis-involvement and crashes is spurious and explained wholly by individual differences in risk-taking (i.e., persons with a risk-taking personality are both more likely to take drugs and to drive recklessly). Finally, unique to this study is the opportunity to assess driving performance along the delta-9-terahydrocannabinol (THC) absorption/elimination curve (ascending versus descending). Because the subjective high and vascular effects of cannabis follow a counterclockwise hysteresis, the same concentration (ng/ml) of THC corresponds with two very different effects depending on whether the user is on the ascending or descending leg of curve. This is extremely valuable information given recent discussions in States legalizing marijuana use and determining drug per se levels for driving. Described herein is a new research methodology-the In-Vivo Driving Impairment Research Method-that has potential to fill gaps in the field's understanding of drug-involved driving. Proposed in this application is a study to test the feasibility of this new method.